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How does calcium trigger contraction in cardiac muscle cells?

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Illustration of a human heart cross-section showing four chambers-left and right atria and ventricles-with labeled major blood vessels. An inset highlights cardiac muscle tissue, showing myocytes and intercalated disks. — human cardiovascular system The human heart is both a pump and a muscle. Its four chambers move blood efficiently, while cardiac muscle cells joined by intercalated disks synchronize each contraction.

In cardiac muscle, the process of excitation-contraction coupling relies on the movement of calcium ions to trigger contraction.12

When a cardiac muscle cell is depolarized during an action potential, calcium channels in the plasma membrane open, allowing a small amount of calcium to enter the cell. This influx stimulates the sarcoplasmic reticulum to release a much larger quantity of stored calcium into the cytosol, increasing the local calcium concentration by nearly 100-fold.23

This rise in calcium concentration is the critical trigger for contraction:
* Conformational Change: Calcium binds to the troponin-tropomyosin system associated with thin actin filaments.12
* Interaction: This binding shifts the conformation of these proteins, which otherwise inhibit the interaction between actin and myosin.23
* Contraction: Once the inhibition is removed, actin and myosin interact to form cross-bridges, leading to muscle contraction.12

Relaxation occurs when the sarcoplasmic reticulum reabsorbs the calcium, lowering its concentration in the cytosol and allowing the troponin-tropomyosin system to return to its inhibitory state.123